Flow transducers are known and function by exploiting various physical effects. Examples of magnetic flow transducers are described in a published international PCT patent application no. PCT/GB2004/001618 (WO 2004/090475). In this published PCT application, a magnetic transducer for measuring a flow of a fluid is elucidated. The transducer is illustrated schematically in FIG. 1 and indicated generally by 10 therein. The transducer 10 includes a tube 20 for guiding a flow F of a fluid. Moreover, the transducer 10 comprises a magnetic assembly indicated generally by 30 for applying a transverse magnetic field 70 in a region of the tube 20 in which the flow F occurs in operation. Furthermore, the transducer 10 includes two electrodes 40a, 40b at the region whereat the magnetic field 70 is applied, the electrodes 40a, 40b being coupled electrically to the fluid in the tube 20 and also connected to inputs of a signal processing unit 90.
The magnetic field 70 when applied is substantially parallel to a first axis orthogonal to a direction of the flow F through the tube 20. The magnetic assembly 30 includes a magnetic field generating component 50 magnetically coupled to pole pieces 60a, 60b so disposed so as to apply the transverse magnetic field 70 as illustrated. Optionally, the magnetic field generating component 50 employs a remnant field property of a magnetic material so as to circumvent a need to expend power in maintaining the magnetic field 70; in such operation, the transverse magnetic field conveniently has a magnitude in an order of milliTeslas.
The electrodes 40a, 40b are operable to sense a potential difference V denoted by an arrow 80 in a second axis, the second axis being substantially orthogonal to the first axis and to the direction of flow F. The potential difference V is generated in operation on account of the fluid flowing in the tube 20 including oppositely-charged free charge carriers, these free charge carriers being affected mutually differently by the magnetic field 70 to generate the potential difference V. The signal processing unit 90 is operable to process the potential difference V, taking into account the transverse field 70, to generate a measure M of the magnitude of the flow F. Integration of the measure M in respect of time provides an indication of a cumulative volume of fluid flowing through the transducer 10.
In order that the transducer 10 is capable of functioning reliably in operation, it is has been found important that the electrodes 40a, 40b are in stable electrical contact with the fluid in the tube 20 and that fluctuating electrochemical offsets do not arise at an interface presented between each of the electrodes 40a, 40b and the fluid. Whereas it has been hitherto conventional practice to employ graphite or corrosion-resistant metal alloy electrodes in related types of fluid flow meters, the aforementioned published PCT application discloses that electrodes comprising a combination of a metal element and a metal halide element operate in a more stable manner than such graphite or corrosion-resistant metal alloy electrodes; the metal halide element provides effectively a more stable electrical contact between the metal element and the fluid. Electrodes fabricated from silver and silver chloride are found to represent a considerable improvement to known approaches utilized in fluid flow transducers.
However, further improvements to those described in the aforesaid published PCT application are required to provide reliable operation over a period of many years when the transducer 10 is, for example, employed in domestic or industrial water flow metering situations. In these metering situations, it is found that an inconveniently large amount of metal halide needs to be included within the transducer 10 to provide reliable operation over the aforesaid period of many years on account of the metal halide element gradually dissolving into the fluid. This inconveniently large amount of metal halide required to provide extended longevity represents a technical problem. A first issue of the problem is that the metal halide element has a tendency to increase series resistance of the electrode 40a, 40b resistance which deleteriously affects noise performance of the transducer 10. Moreover, a second issue of the problem is that incorporation of more metal halide renders the transducer 10 potentially more expensive to manufacture, hence making the transducer 10 less commercially attractive in comparison to known alternative types of fluid flow transducers.